An optical pointing device tracks its movement across a surface by capturing light scattering patterns off the surface. For example, an image of the surface and/or a speckle field from the scattered light can be detected. Images of a surface include dark and light areas that are created by non-uniform scattering of the light due to the roughness of a surface. Speckle fields have dark and light areas that are generated through interference created when coherent light is scattered by a rough surface. The optical pointing device is able to identify relative x-y movements against the surface, e.g., using cross-correlation of consecutive patterns. These x-y shifts are then sent from the pointing device to a computer system to indicate the amount of movement in both the x direction and the y direction.
To generate the signals used in X-Y motion tracking, optical pointing devices typically include a light source such as a light emitting diode (LED) or a vertical cavity surface emitting laser (VCSEL) and an array of light sensors such as a Complementary Metal-Oxide-Semiconductor (CMOS) sensor. In the case of a light beam striking a typical working surface, there are generally two possible channels of light scattering off the surface if other possibilities such as absorption and transmission are excluded.
The first possible channel is known as specular reflection. This type of reflection follows the reflection law in which the angle of reflection equals the angle of incidence. Thus, if the angle of incidence is 45 degrees from the normal to the surface, the angle of the specular reflection will also be 45 degrees. Specular reflection tends to be stronger for surfaces that are relatively smooth and flat.
The second possible channel is known as diffusive scattering. Diffusive scattering is caused by roughness or debris on the surface. Diffusive scattering can scatter the light in many different directions. In the case of a VCSEL beam interacting with a surface, unique surface patterns can be generated by diffusive scattering both due to reflective imaging and due to speckle generation of the scattered light.
The optical pointing devices can work effectively on typical surfaces due to the roughness present on these surfaces, which generates sufficiently strong diffusively scattered light to produce desired patterns. As noted above, these patterns can be due to reflective imaging and/or speckle.
However, pointing devices do not perform well with extremely smooth surfaces, such as marble or glass, which often have very weak diffusively scattered light. As far as these surfaces are concerned under typical configurations in existing implementations of pointing devices, the majority of the light that is incident on them will be dominantly either reflected in the specular direction and/or, in the case of glass, transmitted through the surface. The diffusively scattered light is often too weak to generate a usable signal from reflective imaging and/or speckle, posing great challenges for using a pointing device on such surfaces.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.